All electromagnetic suspension-based systems employ some type of feedback control mechanism to maintain stable levitation. (An interesting exception to the above rule, which has been demonstrated in the laboratory, consists of a permanent magnet suspended below a block of high-temperature superconducting material held below its critical temperature. Stable suspension results from a property of the superconducting state that immobilizes magnetic field lines in the superconducting material.)
A feature of EMS systems that is particularly attractive for maglev vehicles is that the suspension force is essentially independent of speed. Consequently, stable magnetic suspension can be achieved at rest as well as at any other speeds within the system’s design limits. Typically, for vehicle applications, in order to compensate for the force of gravity with an attractive magnetic force, the vehicle and guide way components must be configured so that the onboard magnets are drawn upward toward the undersides of the guide way rails.
The design of the lateral guidance and propulsion systems must, of course, be compatible with the lift system design. A variety of methods for producing lateral guidance in electromagnetic suspension systems have been developed. To increase lateral stiffness, the vehicle-borne magnets are alternately offset from the centerline. Departures from the lateral equilibrium position result in a strong centering force to be exerted on the rail. Another method proposed for combining lift and guidance functions in EMS systems is to place the reaction rails and magnets at an angle so that the magnetic attraction force has both a horizontal component and a vertical component.
In principle, any type of noncontact propulsion system could be used to propel a maglev vehicle. However, in practice, the choice depends on a number of important and often conflicting considerations, including compatibility with the lift and guidance system, power requirements, thrust requirements, operating speed, weight penalties, cost, and environmental constraints. With the exception of some special-purpose applications, linear induction motors (LIMs) and linear synchronous motors (LSMs) have generally been the propulsion means of choice. Both consist of a primary part that generates a traveling magnetic wave, which in turn interacts with a secondary motor part to produce thrust. Air gaps for both types of linear motors are generally approximately 0.5 in. or less in EMS systems. A third type of linear motor uses electronic switching technology to turn electric currents on and off at precisely the right times to create thrust between the primary and secondary motor parts. The latter type, called sequentially excited linear motors, includes locally commutated linear synchronous motors and pulsed linear induction motors.
The stator generally consists of a laminated iron core with transverse slots spaced along its length. Windings are placed in the slots to form a series of coils. In the most common form, three separate windings are placed alternately in the core slots so that when excited with three-phase electric current, a traveling magnetic wave is created. The speed of the traveling wave, called the synchronous speed, is given by the relation, v 5 ¼ 2pf, where p is the distance between stator magnetic poles (the pole pitch) and f is the frequency of excitation in Hertz. The primary power supply is called a variable voltage variable frequency (VVVF) power supply.
When the reaction rail is placed adjacent to the stator, the traveling magnetic wave exposes the rail to time-varying magnetic fields, which in turn cause eddy currents to flow in the rail. Those eddy currents react with the traveling wave and produce a force that has two components: one in the direction of motion, i.e., a thrust force, and another that acts normal to that direction, i.e., a repulsive force. As the speed between the traveling wave and rail (called the slip speed) is increased, the thrust initially increases, reaches a peak value, and then diminishes. The repulsive normal force increases with slip speed.
The long stator option, on the other hand, places the primary on the guideway and the reaction rails on the vehicle, thus reducing vehicle weight and eliminating the need to transfer propulsion power to the vehicle, which results in a lighter, simpler vehicle, but a more complex and expensive guideway. In principle, one could also achieve a better performing system, with greater acceleration and maximum speed. However, continuous generation of eddy currents in the on-board reaction rails is accompanied by ohmic heating, which poses large cooling requirements on board the vehicle.
Whereas low-speed maglev systems have generally used single-sided LIMs, double-sided LIMs using aluminum or copper sheets or shorted-turn coils (coils whose ends are connected together) have been used in applications requiring high acceleration, such as mass drivers and rail guns. Since this type of motor configuration has energized magnetic poles on both sides of the reaction rail, the net normal force is near zero and a good magnetic flux path is provided, leading to a high thrust capability.
The LSM configuration of choice for both electromagnetic suspension and electrodynamic suspension high-speed applications places the primary side on the guide way and the secondary on board the vehicle. This long-stator option has several advantages. It enables the same on-board magnets to be used for lift and as the secondary side of the LSMs. In the case of the German Transrapid system, the laminated iron core of the stator also serves as the reaction rail for the on-board lift magnets. Furthermore, it avoids the problem of transferring propulsion power from the guide way to the vehicle at high speed. It also reduces the weight burden on the vehicle suspension system and places vehicle control at the wayside, eliminating the need for an onboard engineer or driver. To improve efficiency, the primary side of the LSM is normally divided into blocks ranging in length from a few tenths of a mile to several miles. The wayside power control system monitors the position of each vehicle in the system and turns on a block as the vehicle approaches and turns it off after it leaves. The system is designed so that only one vehicle (which may consist of one or more cars linked together as in a train) can occupy a block at a time. This allows short headways while ensuring positive vehicle separation.
